Positive-acting photoresists comprising an alkali-soluble novolac resin and a quinone diazide compound as a sensitizer are well known in the art, as is their use in the manufacture of semiconductor devices. Examples of such positive photoresist formulations are found in U.S. Pat. Nos. 3,666,473, 4,115,128, 4,173,470, 4,377,631, 4,529,682, 4,587,196 and 4,731,319. The teachings of each of these patents are incorporated herein by reference.
Above-referenced U.S. Pat. No. 4,731,319 describes a resin containing a mixture of cresol novolac resins and naphthoquinone diazide sulfonic acid ester as the photosensitive component. Above-referenced U.S. Pat. Nos. 4,377,631, 4,587,196 and 4,529,682 are directed to positive-acting photoresists containing cresol-formaldehyde novolac resins and photosensitive naphthoquinone diazide sulfonyl ester.
In positive-acting photoresists of this type, the novolac resin or mixture of resins is soluble in aqueous alkali solution; however, the quinone diazide compound acts to render the novolac resin insoluble in aqueous alkali solution. The azide quinone compounds are photosensitive, and when exposed to actinic radiation are chemically altered and no longer render the novolac resin insoluble. If a layer of photoresist is exposed to patterned actinic radiation, such as actinic radiation passed through a patterned photomask, exposed portions are rendered soluble in alkaline aqueous developer solution, whereas non-exposed portions remain insoluble in alkaline aqueous developer solution. The exposed device is developed in the alkaline aqueous developer solution, removing exposed portions, while leaving non-exposed portions of the photoresist layer on the device.
The present invention is generally applicable to positive photoresists of the alkaline aqueous-soluble novolac resin/quinone diazide formulation type.
It is, of course important that the pattern of the photomask be reproduced on the layer of exposed and developed photoresist as faithfully as possible. This is particularly true as the push toward miniaturization reduces line size and line spacing. The exposed and developed photoresist layer should have clearly defined, vertical sidewalls on the non-exposed portions of the layer which remain after development.
An impediment to excellent resolution is the effects of reflected light during the exposure step. Often the photoresist is applied to a reflective metal surface. The photoresist may be applied to a topographical surface of a semiconductor device, in which case, corners and edges reflect light in unpredictable manners. The problem is especially severe when the surface is both highly reflective and topographical. Resulting lack of reproductive faithfulness due to reflected light is known as reflective notching. The present invention is directed to the novel use of particular dyes to reduce reflective notching.
The use of dyes for reducing reflective notching has been suggested previously. Unfortunately, many of the known dyes for this purpose also reduce the sensitivity of the photoresist to incident light, typically reducing photospeed by about 2 to 3 times. An extremely important aspect in the selection of a dye is its compatibility with the novolac resin/quinone diazide chemical system. Any incompatibility which results in the development of particulates is unacceptable. Generally, it is desirable that the photoresist formulation be stable against the development of particulates for a period of at least a year. A further consideration for selection of a dye for reducing reflective notching is that it absorb strongly in the visible region of the exposing light sources of the scanner and steppers. Typically, the exposing light is mercury vapor emitting a broadband emission in the ultraviolet region which comprises of G line at 436 nm, H line at 405 nm and I line at 365 nm. Generally, a major output wavelength of a scanner is an I line at 365 nm. Typically, a G line dyed resist has no absorbance at I line with the result that the scanner imaged resist has poor sidewall profile. An I line dye and its concentration was selected such that it has absorbance matching a G line in the resist formulation, thereby enhancing the sidewall profile when imaged on the scanners.
The present invention is directed to the use of very specific coumarin dyes used in positive photoresists of the novolac resin/quinone diazide type. Coumarin dyes have been used in photoresists, and even positive photoresists of the novolac resin/quinone/diazide type previously; however, as will be demonstrated hereinafter, the coumarin dyes of the present invention are surprisingly and unexpectedly superior to coumarin dyes previously used in photoresist. A dye should not significantly reduce photospeed. A dye should be compatible with the photoresist system, including solvents and resins. A dye should minimize reflective notching the dyes of the present invention meet all of these qualifications, and, in one respect or another, are substantially better than any of the coumarin dyes previously used. U.S. Pat. No. 4,626,492 to Eibeck suggests the use of several coumarin dyes available from Eastman Kodak; however, Applicants have tested such coumarin dyes and found them to be deficient in one respect or another.
Coumarins, including coumarins of the formula used in the present invention are found in U.S. Pat. No. 4,147,552 to Sprecht et al. However, the coumarins in the issued patent are used not as dyes to reduce reflective notching, but as photosensitizers in a much different type of photoimageable composition, i.e., in a negative-acting photoresist composition in which there is a release of gas during the photoinitiated reaction.